The present invention relates generally to electric vehicle charging systems, and more particularly, to a power-factor-corrected, single-stage inductive charger system or converter for use in inductive charging of electric vehicle batteries.
The assignee of the present invention manufactures electric vehicles and inductive charging systems to charge the batteries in these electric vehicles. The inductive charging system has two main components. The first component is the inductive charger which is located off the vehicle. The inductive charger conditions the low-frequency utility AC line power, and converts it to high-frequency AC power at the inductive coupler (or plug), which is the output of the inductive charger. The second component is the on-vehicle inductive inlet (or socket) which mates with the inductive coupler of the inductive charger. The high-frequency AC power from the inductive coupler is transformer-coupled on to the vehicle via the inductive inlet. The high-frequency power on the vehicle is subsequently rectified and filtered to generate a DC current to charge the batteries.
The current electric vehicle inductive chargers manufactured by the assignee of the present invention are known as Standard Charge Modules and Convenience Charge Modules. These chargers have two power stages in series to process the power from the utility line to the inductive coupler. The first stage, which is typically a boost-type converter, power factor corrects the current drawn from the rectified low-frequency utility AC line. Such a correction in the wave shape of the utility AC line current maximizes the available utility power and minimizes the utility AC line current and voltage distortion. The first stage additionally converts the rectified utility low-frequency AC to high-voltage DC by filtering the AC using large bulky electrolytic capacitors.
The second power processing stage has two functions: (1) it controls the output power to the battery and (2) conditions the high-frequency AC voltage and current for input to the inductive cable and coupler. The second stage is generally a resonant inverter with MOSFET switches and a series tank composed of an inductor and capacitor. This resonant inverter chops the high voltage DC, produced by the first stage, into high-frequency AC. The high-frequency AC is filtered by the series tank, and fed into a cable which connects to the winding of the inductive coupler, for subsequent transformer-coupling onto the vehicle via the inductive inlet.
The resonant inverter operates at a frequency above the natural frequency of the series tank to enable soft switching of the inverter MOSFETs, resulting in a high efficiency of power transfer. The power transferred from the utility to the battery can be easily regulated by controlling the operating frequency of the resonant inverter. Decreasing the operating frequency will result in increased load current to the battery and vice versa.
The inductive charging for electric vehicles is standardized using the Society of Automotive Engineers Inductive Charge Coupling Recommended Practice, SAE J-1773. SAE J-1773 defines a common electric vehicle conductive charging system and architecture and the functional requirements of the vehicle inlet and mating connector. The inductive charging vehicle inlet defined by the SAE J-1773 practice contains two significant passive elements. These are the transformer magnetizing inductance and a discrete capacitance connected in parallel with the transformer secondary.
When the inductive charger is coupled to the inductive inlet, the series tank of the charger and the parallel tank of the inlet together result in a series-parallel tank. Driving the frequency-controlled resonant inverter into the series-parallel resonant tank which feeds the rectifier and voltage-source battery load results in many beneficial attributes for the charger and inlet: the transformer and cable leakage inductances complement the larger series inductance of the charger; high transformer turns ratio to minimize primary current stress in the charger; buck/boost voltage gain; current-source operation; monotonic power transfer characteristic over a wide load range; throttling capability down to no-load; high-frequency operation; narrow modulation frequency range; use of zero-voltage-switched MOSFETs with slow integral diodes; high efficiency; inherent short-circuit protection, and soft recovery of the output rectifiers.
The first stage of the charger contains many parts, such as a heavy filter inductor, bulky electrolytic storage capacitors, costly power semiconductors, heavy heat sinks, etc, all of which add significant size, weight, and cost to the charger system. The resonant converter also contains many parts. It would therefore be desirable to significantly reduce the number of components in electric vehicle charger systems. It would also be desirable to eliminate the first power processing stage and use only the second stage operating as a single-stage charger. It would also be desirable to have power-factor-correction for the utility interface and SAE J-1773 compatibility for the electric vehicle inductive inlet interface.